Long term photometric monitoring with the Mercator telescope^*,**

Frequencies and mode identification of variable O-B stars

¹ Koninklijke Sterrenwacht van België, Ringlaan 3, 1180 Brussel, Belgium e-mail: peter@oma.be
² Instituut voor Sterrenkunde, K.U. Leuven, Celestijnenlaan 200 B, 3001 Leuven, Belgium
³ Department of Astrophysics, Radboud University Nijmegen, 6500 GL Nijmegen, The Netherlands
⁴ Department of Astronomy and Space Sciences, Faculty of Science, University of Ege, 35100 İzmir, Turkey
⁵ Institut d'Astrophysique et de Géophysique, Université de Liège, Allée du Six Août 17, 4000 Liège, Belgium
⁶ Observatoire de Paris, LESIA, CNRS UMR 8109, 92195 Meudon, France
⁷ INAF - Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy
⁸  Telescope, Calle Alvarez de Abreu 70, 38700 Santa Cruz de La Palma, Spain
⁹ Instytut Astronomiczny, Universytet Wroclawski, Kopernika 11, 51-622 Wroclaw, Poland
¹⁰ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281-S9, 9000 Gent, Belgium
¹¹ Institut für Astronomie, Universität Wien, Türkenschanzstrasse 17, 1180 Wien, Austria
¹² Astronomy Department, Yale University, PO Box 208101, New Haven, CT 06520-8101, USA

Received: 7 August 2006
Accepted: 5 October 2006

Abstract

Aims. We selected a large sample of O-B stars that were considered as (candidate) slowly pulsating B, β Cep, and Maia stars after the analysis of their hipparcos data. We analysed our new seven passband geneva data collected for these stars during the first three years of scientific operations of the mercator telescope. We performed a frequency analysis for 28 targets with more than 50 high-quality measurements to improve their variability classification. For the pulsating stars, we tried both to identify the modes and to search for rotationally split modes.

Methods. We searched for frequencies in all the geneva passbands and colours by using two independent frequency analysis methods and we applied a 3.6 S/N-level criterion to locate the significant peaks in the periodograms. The modes were identified by applying the method of photometric amplitudes for which we calculated a large, homogeneous grid of equilibrium models to perform a pulsational stability analysis. When both the radius and the projected rotational velocity of an object are known, we determined a lower limit for the rotation frequency to estimate the expected frequency spacings in rotationally split pulsation modes.

Results. We detected 61 frequencies, among which 33 are new. We classified 21 objects as pulsating variables (7 new confirmed pulsating stars, including 2 hybrid β Cep/SPB stars), 6 as non-pulsating variables (binaries or spotted stars), and 1 as photometrically constant. All the Maia candidates were reclassified into other variability classes. We performed mode identification for the pulsating variables for the first time. The most probable value is 0, 1, 2, and 4 for 1, 31, 9, and 5 modes, respectively, including only 4 unambiguous identifications. For 7 stars we cannot rule out that some of the observed frequencies belong to the same rotationally split mode. For 4 targets we may begin to resolve close frequency multiplets.